A vital cellular component, the actin cytoskeleton influences and molds the shape, motility, and organization of eukaryotic cells. The force generated by rapid actin assembly and disassembly is required for essential cellular processes such as spreading, movement, cell division, and endocytosis. The Arp2/3 complex, one of three known actin nucleators, nucleates actin filaments, in response to cellular signals, from the sides of pre-existing filaments, forming branched networks of actin filaments in the cell. While it is known that ATP binding is carried out by the Arp2/3 complex on the Arp2 and Arp3 subunits, the exact function of ATP binding and hydrolysis by these subunits is unclear. In the cell, nucleotide hydrolysis has many functions and is a ubiquitous and recurring device that cells use for regulation. Studying the ATP hydrolysis mechanism of the Arp2/3 complex is a critical avenue to understanding the regulation of actin in the cell. [unreadable] [unreadable] I propose a series of in vitro and in vivo experiments aimed at explaining the role of ATPase activity by the Arp2/3 complex. By making mutations and using purified proteins in vitro, combined with an in vivo S2 Drosophila cell system, I will be able to link in vitro observations to in vivo cellular phenotypes. [unreadable] [unreadable] Cellular migration, contraction, and division, are processes that occur via the actin cytoskeleton. In exploring the regulation of actin polymerization, this research will contribute to our understanding of cellular mechanisms that play roles in the etiology of such illnesses as heart disease and cancer. [unreadable] [unreadable] [unreadable]